Pressure-induced phonon frequency shifts in transition-metal nitrides

We report the experiments on the high-pressure phonon spectra of transition-metal nitrides HfN, ZrN, and NbN, obtained by Raman-scattering measurements. Two pronounced bands, which are related to the acoustic part at low-frequency around 2000.3em0excm^-1 and the optical part at high-frequency around 5500.3em0excm^-1 of the phonon spectrum, respectively, shift to high-frequency values with increasing pressure. An analysis of the results allows us to reproduce the experimental pressure dependence of the superconducting transition temperature Tc of ZrN and NbN. We also predict the pressure effect on Tc in HfN.